LONDON: Russia's Nikolay Davydenko stunned Roger Federer to reach the final of the ATP World Tour Finals with a 6-2, 4-6, 7-5 victory over the world number one on Saturday.

Federer had never lost to Davydenko in 12 previous meetings, but the Wimbledon and French Open champion was unable to extend that run in the semi-finals at London's O2 Arena.

Davydenko, who will face Argentine Juan Martin Del Potro in Sunday's final, had only qualified for the last four late on Friday and he was back on court just 14 hours later.

But the Russian showed no signs of fatigue as he booked his place in the final for the second successive year.

Fifth seed Del Potro meanwhile overcame Swedish eighth seed Robin Soderling 6-7 (1/7), 6-3, 7-6 (7/3) in an equally tight second semi to earn his place in the trophy match despite his rival firing down 16 aces.

Davydenko said: "I think all my family, everybody who supports me, have waited for this moment because I beat everyone in top 10 except Federer. It's a really good feeling.

"In 12 matches he beat me. I was thinking 'Not again, please'. But then I have better concentration. Then I start to be lucky.

"I don't think now I'm number one because I beat Federer, but I know now, if I come to a match and play against number one, number two, number three in the world, I have a chance. That's really good confidence for next year."

Four-time winner Federer admitted his poor start to the match was decisive.

"It's disappointing, not to lose against him, just to lose in the semis," he said.

"Coming so far in a tough group, in a tough tournament, I had hoped to get through to the final and maybe win again.

"But I missed the start again and I guess that's what cost me the match in the end. I tried everything to have a good start but I wasn't able to do it again.

"In the second set it was more a story of just trying to hang on, get through to the third. As the match wore on, I actually started to feel better and better.

"I was a couple of points away but just couldn't get it done. He did well. I thought he played strong throughout, even though I don't think it was our best match we ever played against each other."

Federer could not take advantage of two break points early on and then played a terrible game, serving two double faults and throwing in two more unforced errors to hand Davydenko the first break.

Another double fault and a netted forehand gave Davydenko - beaten in last year's final by Novak Djokovic - a shock 4-1 lead.

The Swiss star got one of the breaks back immediately, but the respite was only temporary and Davydenko made it three breaks in a row before sealing the first set.

Federer was in danger of being blown away and he gradually found some rhythm in the second set. A series of superb winners seized the initiative and he broke at 5-4 to level the match.

He had the advantage of serving first in the decider and at 4-5 the match came alive.

A lucky net cord gave him the opening point and an outrageous shot off a Davydenko smash put Federer in sight of the final.

But he could not take his chance as Davydenko held his nerve to level at 5-5.

And the Russian capitalised fully by creating a break point in the next game, which he took with a stunning backhand return.

Two big serves took Davydenko to 30-0 but Federer was not finished, a brilliant rally resulting in a break point.

This time Davydenko blasted a forehand winner and one big serve later he was at match point, which he took when Federer netted a forehand.

Del Potro meanwhile roared back after a sluggish opening set tiebreak where he took a solitary point after missing four break chances across the set.

The US Open champ then upped his game to level the encounter before prevailing in the deciding breaker with a 12th ace.